Method for producing time marks at any points on moving components

ABSTRACT

The invention relates to a method for generating time markers of arbitrary points on moving components, and also to an optical trigger device for carrying out the method, as illustrated in FIG. 1. According to the invention, light from a broadband light source ( 1 ) with a correspondingly short coherence length is split into two paths, namely a measurement light path ( 3 ) and a reference light path ( 4 ), and is coupled into two optical waveguides ( 5, 6 ), the light of the reference light path ( 4 ) is at least partially coupled back and the light of the measurement light path ( 3 ) is focussed at a measurement point ( 11 ) and reflected by the component passing through the focus ( 11 ), wherein the light distances in the measurement light path ( 3 ) up to the focus ( 11 ) and in the reference light path ( 4 ) up to the coupling-back plane ( 12,18 ) are of the same length within the coherence length of the radiated light, wherein the coupled-back light from the reference light path ( 4 ) and the measurement light path ( 3 ) is brought to interference and detected by a detector ( 14 ), and wherein a short modulation event occurs at the detector ( 14 ) when the component ( 13 ) passes through the focus ( 11 ) (FIG.  1 ).

[0001] The invention relates to a method for generating time markers of arbitrary points on moving components, and also to an optical trigger device for generating such time markers.

[0002] In the development of modern turbine engines, for example, oscillation measurements on the highly loaded blades of the rotors are very important. Critical oscillations must be identified in good time. At the same time, the measurements supply data for the design programs and for estimating the service life of the blades. To date, the measurements have, in the majority of cases, been carried out using strain gauges, which is associated with considerable costs and lead times on account of the necessity for complex cabling which is resistant to centrifugal force and a telemetry system for signal transmission.

[0003] Optical measurement methods have been developed in order to avoid the difficulties outlined above. In this case, trigger probes are mounted at different peripheral positions in the housing of the component to be measured, such as e.g. a rotor blade of a turbine engine, with which it is possible to measure the travel times of the blades between the probes. In the absence of oscillation, the travel times are determined by rotational speed, rotor radius and peripheral position of the probes. Oscillations lead to a change in the travel times since the blades pass the probes earlier or later—depending on the instantaneous phase angle of the oscillatory movement. Amplitude and frequency of the oscillations are determined from the travel time deviations from the undisturbed values.

[0004] In order to generate the time or position signals of the blades, probes are built into the housing above the rotor, which probes supply a signal when the tip of a blade passes through the measurement region of the probes. Time markers are derived from the resultant analogue signals by electronic means, which time markers are correlated with a defined position of the blade tips. In special cases, the probes can also project into the flow channel. Trigger signals are then derived from the leading or trailing edge of the blades.

[0005] The known probe configurations have a significant disadvantage in that time markers can be generated only at the edges of the blades, normally at a location on the blade tip. Oscillatory forms actually having a node there are not detected. Moreover, when the probes are built into the flow channel for measurement on the leading and trailing edge of the blades, the flow is considerably disturbed.

[0006] It is an object of the invention, therefore, to provide a method for generating time or precision markers which can generate time markers at arbitrary points on moving components. The intention is to enable a largely flexible application and, in particular, free position-ability of the probes. Furthermore, the intention is to propose an apparatus for carrying out the method.

[0007] The invention proposes a method for generating time markers of arbitrary points on moving components, wherein light from a broadband light source with a correspondingly short coherence length is split into two paths, namely a measurement light path and a reference light path, and is coupled into two optical waveguides, the light of the reference light path is at least partially coupled back and the light of the measurement light path is focussed at a measurement point and reflected by the component passing through the focus, wherein the light distances in the measurement light path up to the focus and in the reference light path up to the coupling-back plane are of the same length within the coherence length of the radiated light, wherein the coupled-back light from the reference light path and the measurement light path is brought to interference and detected by a detector, and wherein a short modulation event occurs at the detector when the component passes through the focus.

[0008] The method according to the invention uses a broadband light source. The light emitted by this light source is split into two paths by a beam splitter. One portion of the light is fed into a measurement optical waveguide and the other part is fed into a reference optical waveguide. The light of the measurement light path emerges at the end of the optical waveguide and is focussed in front of the optical waveguide. The focus constitutes the measurement point. The light of the reference light path is at least partially coupled back. This can either be done directly at the end face of the optical waveguide or be achieved by means of a separate mirror.

[0009] If a moving component passes through the focus of the measurement light path, part of the light is reflected and coupled back into the measurement optical waveguide. The coupled-back portions of the measurement light path and of the reference light path are then brought to interference and detected by a detector.

[0010] At the detector, modulation of the light intensity occurs as a result of the interference of the two light paths when the light distances in the measurement light path up to the focus and in the reference light path up to the coupling-back plane are of the same length within the coherence length of the radiated light. Therefore, the arrangement is to be calibrated in such a way that this condition is met precisely in the focus region of the measurement light path. If a component then passes through the light beam of the measurement light path at the focus, the intensity of the reflected light coupled back into the optical waveguide reaches a maximum. While the maximum is traversed, modulation of the light intensity momentarily occurs at the detector in accordance with the shortness of the coherence length of the light source, the modulation frequency being dependent on the wavelength of the light used and on the speed of the component. This modulation event can be used to derive a trigger pulse of high temporal accuracy.

[0011] A Michelson interferometer is preferably used as the beam splitter and for interference of the coupled-back light portions, it also being possible to use other interferometric arrangements.

[0012] The invention furthermore proposes an optical trigger device for generating time markers of arbitrary points on moving components, comprising a broadband light source with a correspondingly short coherence length, means for splitting the light from the light source into two paths, namely a measurement light path and a reference light path, which each comprise an optical waveguide and means for feeding the light into the respective optical waveguides, means for focussing the light emerging from the measurement optical waveguide at a measurement point, means for coupling back the light of the reference light path, means for interference of the coupled-back light of the two paths, and a detector for detection of the interfering, coupled-back light.

[0013] The optical trigger device according to the invention can be used to generate time markers of arbitrary points on moving components. The trigger device according to the invention comprises a broadband light source. The light emerging from this light source is split into two paths, namely a measurement light path and a reference light path. The light of the measurement light path is fed into a measurement optical waveguide and the light of the reference light path is fed into a reference optical waveguide. Both optical waveguides lead to a probe. In this probe, the light emerges from the measurement optical waveguide and is focussed at a measurement point. The light of the reference light path is at least partially coupled back into the reference optical waveguide again. This can be achieved either by reflection at the boundary surface of the optical waveguide or by means of a separate mirror.

[0014] If a component to be measured, for example a rotor blade of a turbine engine, passes through the focus of the measurement light path, the light is partially reflected and coupled back into the measurement optical waveguide. The coupled-back light portions are combined from the two paths and then interfere with one another. A detector detects the interfering, coupled-back light of the two paths.

[0015] At the detector, modulation of the light intensity occurs when the two distances in the measurement and reference light paths are of the same length within the coherence length. Therefore, the apparatus is preferably initially to be calibrated in such a way that this condition is met precisely at the focus in front of the probe. For this purpose, in a preferred development of the device according to the invention, means are provided which enable the optical path length of the reference light path and/or of the measurement light path to be adjusted. This can be done for example by means of an axial adjustment possibility of the coupling-in optics for coupling the split light from the light source into the optical waveguides.

[0016] If the component to be measured passes through the focus of the measurement light path, the intensity of the coupled-back light reaches a maximum. As a result of the tuning of the arrangement, modulation of the light intensity briefly occurs at the detector during the maximum, the length of the modulation event being dependent on the coherence length of the light source used and the modulation frequency being dependent on the wavelength of the light used and on the speed of the component.

[0017] A Michelson interferometer is preferably used for splitting the light from the light source into two paths and for bringing about the interference of the coupled-back light portions. As an alternative, e.g. a fibre interferometer can be used.

[0018] The end of the measurement optical waveguide and the focussing optics are preferably combined in a probe in which the reference optical waveguide can also end. The measurement probe is easily positioned. The position of the focus of the measurement light path is preferably adjustable in order to enable adaptation to the space conditions available during the measurement. In order to achieve optimum adaptation to the component to be measured and its accessibility, the light beam from the reference light path can emerge obliquely from the probe. The probe can be provided with an oblique window for this purpose.

[0019] In a development of the apparatus according to the invention, a plurality of optical waveguides for simultaneously generating trigger signals of a plurality of points on a component surface can be built into a probe.

[0020] The two optical waveguides of the measurement and reference light paths preferably run in close contact over their entire length. As a result, optical length changes caused by temperature fluctuations, oscillations and similar effects occur to the same extent in both paths and do not cause a disturbance.

[0021] The reference light path is preferably provided with means for attenuating the light intensity in order to enable the signal intensity to be tuned to that of the measurement path. This can be done by means of an attenuator disc.

[0022] A fibre interferometer can be used for splitting the light between the two paths and subsequently uniting it.

[0023] Electronic means can be used to derive an accurate time marker from the high-frequency output signal of the detector.

[0024] In the apparatus, a plurality of optical waveguides for simultaneously generating trigger signals of a plurality of points on a component surface can be built into a probe.

[0025] The invention is explained in more detail below with reference to the accompanying figures:

[0026]FIG. 1 shows the diagrammatic construction of an optical trigger device according to the invention;

[0027]FIG. 2 shows the diagrammatic construction of an optical trigger device according to the invention with a fibre-optic beam splitter;

[0028]FIG. 3 shows an output signal of the detector and the possible derivation from it of an accurate time marker.

[0029]FIG. 1 illustrates a broadband light source 1 which emits in the visible region. This is a light-emitting diode having the wavelength λ=630 nm in the present case. The bandwidth is Δλ=50 nm. The coherence length l_(k) then results from l_(k)=λ²/Δλ. The coherence length is thus about 8 μm. The light from the light source is radiated into a Michelson interferometer provided with a beam splitter 2. The beam splitter splits the light from the light source 1 into a measurement light path 3 and a reference light path 4. The light is firstly collimated and is fed after splitting by the lens systems 7 and 8 into the respective optical waveguides 5, 6. The optical waveguides are kept in close contact over as much of their length as possible. As a result, optical length changes caused by temperature fluctuations, oscillations and other effects occur equally in the two paths and do not cause a disturbance. In the probe 9, the light emerges from the measurement optical waveguide 5 and is focussed at the focus 11 by a lens system 10. The light of the reference optical waveguide 6 is at least partly coupled back at the end 12 of the said waveguide.

[0030] In order to generate a time marker, a component 13 to be measured, for example a rotor blade of a turbine engine, is moved through the focus 11. As a result, the light is partially reflected and coupled back into the measurement optical waveguide 5. The coupled-back light again impinges on the beam splitter 2 of the Michelson interferometer. The light interferes with the coupled-back light of the reference light path 4 and is detected by the detector 14.

[0031] At the detector 14, modulation of the light intensity occurs when the distances of the two light paths 3, 4 are the same within the coherence length. Therefore, the arrangement is initially to be calibrated in such a way that this condition is met precisely at the focus 11 in front of the probe 9. For this purpose, in the construction illustrated here, an axial adjustment possibility A1, A2 is provided in order to have the greatest possible freedom with the position of the focus 11.

[0032] In order to be able to adapt the signal intensity of the reference light path 4 to the signal intensity of the measurement light path 3, the reference light path 4 is provided with an attenuator disc 15.

[0033]FIG. 2 illustrates an optical trigger device which operates in the same way, in principle, and in which a fibre-optic beam splitter 2 is used. The light from the light source 1 is coupled into a fibre and, at the fibre-optic beam splitter 2, split into a measurement light path 3 and a reference light path 4. The measurement optical waveguide 5 leads to a probe 9, which is constructed in the manner explained and illustrated in FIG. 1. The reference optical waveguide 6 leads to an adjusting unit 16, in which the light beam is collimated by suitable optics 17 and reflected at a mirror 18.

[0034] The mirror is provided with an adjustment possibility A3. This enables the effective optical path length of the reference light path 4 to be adjusted. The calibration of the path lengths of the two light paths which is necessary before time markers are generated can thus be achieved despite the lack of an adjustment possibility on the fibre-optic beam splitter 2.

[0035]FIG. 3 illustrates the signal detected by the detector and the signals produced after exemplary processing. In order to generate a time marker, the component to be measured, for example a turbine blade, moves through the focus of the measurement light path. As a result, the light is reflected and partially coupled back into the measurement light path. That leads to modulation of the signal to be detected at the detector. Given a wavelength of 630 nm, for example, and peripheral speeds of a turbine blade in the range between 10 and 500 m/s, this leads to modulation frequencies of from 30 to 1600 MHz. After passing through a high-pass filter, the signal has the form illustrated in FIG. 3a. The signal illustrated in FIG. 3b is then achieved after rectification. By using a low-pass filter, the signal assumes the form illustrated in FIG. 3c. A time marker V, as is illustrated in FIG. 3d, can be formed e.g. after differentiation of the signal from FIG. 3c at the negative zero crossing of the resulting signal.

[0036] List of Reference Symbols

[0037]1 Light source

[0038]2 Beam splitter

[0039]3 Measurement light path

[0040]4 Reference light path

[0041]5 Measurement optical waveguide

[0042]6 Reference optical waveguide

[0043]7 Coupling-in optics

[0044]8 Coupling-in optics

[0045]9 Measurement probe

[0046]10 Focussing optics

[0047]11 Focus

[0048]12 Reflection surface

[0049]13 Turbine blade

[0050]14 Detector

[0051]15 Attenuator disc

[0052]16 Adjusting unit

[0053]17 Coupling-in optics

[0054]18 Mirror 

1. Method for generating time markers of arbitrary points on moving components (13), wherein light from a broadband light source (1) with a correspondingly short coherence length is split into two paths, namely a measurement light path (3) and a reference light path (4), and is coupled into two optical waveguides (5, 6), the light of the reference light path (4) is at least partially coupled back and the light of the measurement light path (3) is focussed at a measurement point (11) and reflected by the component passing through the focus (11), wherein the light distances in the measurement light path (3) up to the focus (11) and in the reference light path (4) up to the coupling-back plane (12, 18) are of the same length within the coherence length of the radiated light, wherein the coupled-back light from the reference light path (4) and the measurement light path (3) is brought to interference and detected by a detector (14), and wherein a short modulation event occurs at the detector (14) when the component (3) passes through the focus (11).
 2. Method according to claim 1, characterized in that a Michelson interferometer is used for splitting the light and for interference of the coupled-back light.
 3. Optical trigger device for generating time markers of arbitrary points on moving components (15) in accordance with the method according to claim 1 or 2, comprising a broadband light source (1) with a correspondingly short coherence length, means (2) for splitting the light from the light source (1) into two paths, namely a measurement light path (3) and a reference light path (4), which each comprise an optical waveguide (5, 6) and means (7, 8) for feeding the light into the respective optical waveguides (5, 6), means (10) for focussing the light emerging from the measurement optical waveguide (5) at a measurement point (11), means (12, 18) for coupling back the light of the reference light path (4), the light distances in the measurement light path (3) up to the focus (11) and in the reference light path (4) up to the coupling-back plane (12, 18) being of the same length within the coherence length of the radiated light, means (2) for interference of the coupled-back light of the two paths (3, 4), and a detector (14) for detection of the interfering, coupled-back light, at which a short modulation event occurs when the component (13) passes through the focus (11).
 4. Optical trigger device according to claim 3, characterized in that the optical path length of the measurement light path (3) and/or of the reference light path (4) is adjustable.
 5. Optical trigger device according to claim 3 or 4, characterized in that the means (2) for splitting and interference of the light are a Michelson interferometer.
 6. Optical trigger device according to one of claims 3 to 5, characterized in that the position of the focus (11) of the measurement light path (3) is adjustable.
 7. Optical trigger device according to one of claims 3 to 6, characterized in that the light intensity of the reference light path (4) is adjustable.
 8. Optical trigger device according to one of claims 3 to 7, characterized in that the trigger device comprises a plurality of measurement light paths (3) for simultaneously generating trigger signals of a plurality of points on moving components. 